Exim config change (connection_max_messages) to improve delivery to Yahoo, AOL

As a user and hosting provider, I suggest some changes to improve delivery to Yahoo, AOL, and other domains that use Verizon anti-spam system.

Apparently the default CPanel/Exim settings lead to frequent outgoing mail delays if any users or Mailman lists are sending to large number of users on these domains. This shows up as a possibly misleading message in the Exim logs: "421 [IPTS04] Messages from xx.xx.xx.xx temporarily deferred due to unexpected volume or user complaints"

This problem can often be resolved by modifying the transports, remote_smtp and dkim_remote_smtp, to specify connection_max_messages = 20 and it also appears helpful to set hosts_max_try_hardlimit as a small number.

I'm not aware of any way to make these changes using CPanel/WHM interface. The edits can done in /etc/exim.conf or /usr/local/cpanel/etc/exim/replacecf/dkim/remote_smtp but CPanel sometimes overwrites these files.

I suggest that CPanel by default should specify connection_max_messages = 20, and also allow users to configure hosts_max_try_hardlimit in WHM.

Related information (including some other suggestions for resolving the delivery problem):

https://forums.cpanel.net/threads/yahoo-defers-mail-from-cpanel.675909/

https://www.rackaid.com/blog/yahoo-defers-cpanel-mail/

https://forums.cpanel.net/threads/configure-connection_max_messages-for-smtp-connections.619391